An electromagnetic wave propagating along north lies its electric field vertically upward. The magnetic field vector points towards

As the electromagnetic wave is the crossed field of electric and magnetic waves, so the direction of propagation of EM wave is the direction of vector $E \times B$. Here $E$ is upward and $(\mathrm{E} \times \mathrm{B})$ is towards north. So, from right hand thumb rule B will be along east.